(1) Technical Field
The present invention relates to a fabrication technique for a micro-electro-mechanical system (MEMS) micro relay switch to increase the reliability, yield, and performance of its contacts. Specifically, the invention relates to a planarization process for the cantilever beam, surface passivation of the substrate, and a unique design of the metal dimple for making a reproducible and reliable contact.
(2) Discussion
Today, there are two types of MEMS switches for RF and microwave applications. One type is the capacitance membrane switch known as the shunt switch, and the other is the metal contact switch known as the series switch. Besides the two types of switches mentioned above, designs can vary depending on the methods with which the switches are actuated. Generally, switch designs are based on either electrostatic, thermal, piezoelectric, or magnetic actuation methods.
The metal contact series switch is a true mechanical switch in the sense that it toggles up (open) and down (close). One difference among the metal contact switch designs is in their armature structure. For example, switches from Sandia National Labs and Teravita Technologies use an all metal armature. MEMS switches from Rockwell use an armature composed of a metal layer on top of an insulator and switches from HRL Laboratories, LLC use an insulating armature having a metal electrode that is sandwiched between two insulating layers. Because of the difference in armature designs, metal contacts in these devices are all fabricated differently; however, in each of these designs the metal contacts are all integrated with part of the armature. The performance of these switches is mainly determined by the metal contact and the armature design. One important issue, occurring when the metal contact is part of the armature, relates to the fabrication process, wherein performance may be sacrificed if the contact is not well controlled.
U.S. Pat. No. 6,046,659 issued Apr. 4, 2000 to Loo et al. (herein after referred to as the “Loo Patent”) discloses two types of micro-electro-mechanical system (MEMS) switches, an I-switch and a T-switch. In the “Loo Patent”, both the I and T-MEMS switches utilize an armature design, where one end of an armature is affixed to an anchor electrode and the other end of the armature rests above a contact electrode.
FIG. 1A depicts a top view of a T-switch 100 as disclosed in the prior art. A cross-section of the switch shown in FIG. 1A is shown in FIGS. 1B and IC. In FIG. 1B the switch is in an open position, while in FIG. 1C, the switch is in a closed position. In this aspect, a radio-frequency (RF) input transmission line 118 and a RF-output transmission line 120 are disposed on the substrate 114, shown in FIG. 1B. A conducting transmission line 128 is disposed across one end of an armature 116, allowing for connection between the RF-input transmission line 118 and the RF-output transmission line 120 when the switch is in the closed position. One skilled in the art will appreciate that the cross-section only shows the contact of the armature 116 with the RF-output transmission line 120, since the contact of the armature 116 with the RF-input transmission line 118 is directly behind the RF-output transmission line 120 when looking at the cross-section of the switch. Thus, for ease of explanation, FIGS. 1B and 1C will be discussed emphasizing the RF-output transmission line 120; however, the same explanation also holds for contacting of the RF-input transmission line 118. Further, one skilled in the art will appreciate that the RF-input and RF-output transmission lines are labeled as such for convenience purposes only and are interchangeable.
When the switch is in an open position, the transmission line 128 sits above (a small distance from) the RF-input transmission line 118 and the RF-output transmission line 120. Thus, the transmission line 128 is electrically isolated from both the RF-input transmission line 118 and the RF-output transmission line 120. Furthermore, because the RF-input transmission line 118 is not connected with the RF-output transmission line 120, the RF signals are blocked and they cannot conduct from the RF-input transmission line 118 to the RF-output transmission line 120.
When the switch is in closed position, the conducting transmission line 128 is in electrical contact with both the RF-output transmission line 120, and the RF-input transmission line 118. Consequently, the three transmission lines 120, 128, and 118 are connected in series to form a single transmission line in order to conduct RF signals. The “Loo Patent” also provides switches that have conducting dimples 124 and 124′ attached with the transmission line 128 which define metal contact areas to improve contact characteristics.
FIG. 1B is a side view of a prior art micro-electro-mechanical system (MEMS) switch 100 of FIG. 1A in an open position. A conducting dimple 124 protrudes from the armature 116 toward the RF-output transmission line 120. The transmission line 128 (shown in FIG. 1A) is deposited on the armature 116 and electrically connects the dimple 124 associated with the RF-output transmission line 120 to another dimple 124′ associated with the RF-input transmission line 118.
FIG. 1C depicts the MEMS switch 100 of FIG. 1A in a closed state. When a voltage is applied between a suspended armature bias electrode 130 and a substrate bias electrode 122, an electrostatic attractive force will pull the suspended armature bias electrode 130 as well as the attached armature 116 toward the substrate bias electrode 122, and the (metal) contact dimple 124 will touch the RF-output transmission line 120. The contact dimple 124 associated with the RF-input transmission line 118 will also come into contact with the RF-input transmission line 118, thus through the transmission line 128 (shown in FIG. 1A) the RF-input transmission line 118 is electrically connected with the RF-output transmission line 120 when the switch is in a closed position. Note that in the FIG. 1A, the armature 116 is anchored to the substrate 114 by an anchor 132 and that bias input signal pads 134 and 136 are provided for supplying power necessary for closing the switch 100.
FIG. 2A depicts a top view of an I-switch 200 as disclosed in the prior art. FIG. 2B depicts a direct current (DC) cross-section of the switch 200 while, FIG. 2C depicts a RF cross-section of the switch 200. In FIG. 2B, a DC signal is passed from the DC contact 220 through an anchor point 222 and into a DC cantilever structure 224. A substrate bias electrode 226 is positioned on the substrate 114. As a DC bias is applied to the DC contact 220 and the substrate bias electrode 226, the DC cantilever structure 224 is pulled toward the substrate 114, causing the RF cantilever structure 215 (shown in FIG. 2C), shown in FIG. 2A, to also be deflected toward the substrate 114. FIGS. 2D and 2E depict the switch 200 in the closed position from the same perspectives as shown in FIGS. 2B and 2C, respectively.
FIG. 2C depicts the RF cross-section of switch 200. The RF-input transmission line 210 passes through anchor point 214 and into the RF cantilever structure 215. The metal dimple 216 protrudes from the RF cantilever structure 215. For ease of explanation the RF cantilever structure 215 and the DC cantilever structure 224 are described herein as two separate structures; however, one skilled in the art will appreciate that these two structures are typically made of one piece of material. The metal dimple 216 provides an electrical contact between the RF-input transmission line 210 and the RF-output transmission line 212. As discussed above, when a DC bias is applied to the DC contact 210 and the substrate bias electrode 226 (shown in FIG. 2B), the RF cantilever structure 215 is deflected toward the substrate 114. The deflection of the RF cantilever structure 215 toward the substrate 114 provides an electrical path between the RF-input transmission line 210 and the RF-output transmission line 212. FIGS. 2D and 2E depict the switch 200 in the closed position from the same perspectives as shown in FIGS. 2B and 2C, respectively. Note that in FIG. 2A the path shown in FIGS. 2B and 2D is depicted between 200b and 200b′ in and that the path shown in FIGS. 2C and 2E is depicted between 200c and 200c′. 
The process of forming the dimple on the armature requires carefully controlled etching times. The dimple is typically formed by first depositing an armature on top of a sacrificial layer. Then a hole is etched through the armature into the sacrificial layer immediately above the RF-input and/or output transmission line. The dimple is then deposited to fill the etched hole. In this case, the height of the dimple depends on the depth of the etching through the hole into the sacrificial layer. This etching process is monitored by time. The time required to obtain the proper etch depth is mainly determined from trial and error etching experiments. Because the etching is a time-controlled process, the etch depth may vary from run to run and from batch to batch depending upon the etching equipment parameters. Thus, the quality of the contact will vary from run to run. For example, if the dimple is made too shallow, the contact will be less optimal. In the worst case, if the dimple is made too deep, a joint between the dimple and the input transmission line may form, ruining the switch. Therefore, there is a need for a switch and a method of producing a switch that may be manufactured consistently to make large volume manufacturing runs economically feasible.